Coating compositions can be used on a wide range of substrates for a variety of purposes. Coating compositions, for example, can be used to form coatings that impart weather resistance, wear resistance, protection against moisture; and cosmetic benefits. So-called gel coats are one kind of coating formed from coating compositions. For instance, gel coats can provide surfaces with high initial and extended gloss for long-lasting visual appeal or otherwise provide a high quality finish on the visible surface of a product. Gel coats can also be used to present desired color characteristics or other visual effects such as fluorescent, pearlescent, iridescent, metallic reflective, non-reflective, and/or retroreflective effects, or the like. Gel coats are used on a wide variety of substrates, including surfaces of marine vessels, motor vehicles, air craft, recreational vehicles, pools, countertops, appliances, bathroom fixtures, buildings and other man-made structures, sports equipment, and the like.
Gel coats are derived from curable, fluid compositions that are applied onto supporting surfaces and cured to form solid coatings. In some instances, such coatings are formed directly on the substrate to be coated. In other instances, the coating is formed against a female mold in a so-called in mold coating (“IMC”) processes. At least a portion of the substrate is then fabricated in situ against the at least partially cured coating while supported by the female mold. The mold surface typically may be a specialized “tooling” gel coat that has been formulated for the construction of molds and reinforced with a low shrink unsaturated polyester resin blend with excellent thermocycling properties. Other mold surfaces, which may be heated to a desired temperature, such as a nickel-plated steel surface, are used for IMC processes.
Coating compositions based upon unsaturated polyester resins are known. Such compositions often are derived from an unsaturated polyester resin diluted with styrene and/or methyl methacrylate monomer(s). Such gel coat compositions may also contain other reactive monomers such as acrylates, methacrylates, and the like. Typical gel coat compositions also often include fillers/extenders such as aluminum trihydrate, calcium carbonate, and the like. Other ingredients commonly used in typical gel coats include application promoters, air release additives, bactericides, fungicides, antistatic agents, antioxidants, rheology agents, wetting agents, UV stabilizers, combinations of these, and the like.
Coating compositions based upon polyisocyanates and so-called polyaspartate (or polyaspartic) resins containing secondary amine groups also are known and have been described in U.S. Pat. Nos. 5,561,214; 5,126,170; 5,236,741; 5,397,930; U.S. Pat. Pub. Nos. 2005/0059792; 2002/0002300; 2009/0226644; and Canadian Application 2,111,927. These coating compositions show great promise, but technical issues remain. Curing challenges are of particular concern in in-mold coating (IMC) applications, where follow through cure after initial curing has been poor.
These amine-based compositions also are susceptible to foaming during curing reactions. This can lead to undue porosity in the cured film. Although anti-foaming agents can be used to alleviate foaming to some degree, excessive use of anti-foaming agents can adversely impact the properties of the resultant coatings.
Another challenge in IMC applications is the formation of surface defects commonly referred to as “fish eyes”. These defects comprise discontinuities in the coating. Fish eyes may have a greater tendency to form when thin (e.g., 2-6 mil thick) coatings are sprayed onto a mold, especially a waxed mold due at least in part to surface tension effects. Another challenge in IMC applications is a need to re-wax the mold after too few “pulls” of articles from the mold, even when using the most advanced mold waxes.